In dielectric barrier discharge plasma sources plasma is generated at the surface of a dielectric layer. Electrodes are used to generate a high frequency electric field with a component normal to that surface, which gives rise to ionization of gas present at the surface of the dielectric layer thus generating the plasma.
There are a number of different types of DBD plasma sources. In the conventional configuration shown in FIG. 1a, the electric field is generated between a first electrode 10 and second electrode 12 with surfaces that face each other. A dielectric layer 14 is provided on the surface of at least one of the electrodes, leaving a gas filled gap 16 between the dielectric layer and the other electrode. This type of DBD plasma source is called “volume” DBD plasma source, because the plasma is created in the volume of the gap.
An article titled “Atmospheric pressure plasma of dielectric barrier discharges” by A. Chirikov et g, published in Pure Appl. Chem., vol 77 pp 487-495 (2005) discusses a plasma source of this type and discusses its basic physics. The article by Chirikov et al gives glass, quartz, ceramics and polymer as examples of dielectrics, and more specifically a steel tube coated with enamel.
Electric breakdown of the dielectric layer can result in damage to the plasma source and to malfunction. Superficial local damage may reduce functionality even if it does not lead to complete failure. The susceptibility of the dielectric layer is conventionally characterized by its nominal electric breakdown field strength. For an ideal dielectric layers breakdown is avoided if the electric field in the plasma source is kept below the nominal electric breakdown field strength of the dielectric layer. At the same time use in a plasma source defines a minimum needed value of the electric field: the electric field should be sufficiently high not only to excite the plasma, but also to ensure full coverage of the surface of the dielectric layer.
The dielectric material must be chosen to ensure that the nominal electric breakdown field strength exceeds the minimum needed value of the electric field. In reality, however imperfections of the dielectric layer and dynamic effects give rise to a risk of breakdown even if the nominal electric breakdown field strength is sufficiently high. A safety margin is needed to achieve a reasonable expected operational lifetime of the plasma source.
Additionally, if the electric field strength varies as a function of position, the nominal electric breakdown field strength needs to exceed the minimum needed value at the positions where the electric field is highest. This is not an issue in the volume DBD plasma source of FIG. 1a, because the electric field between flat plates is uniform and equal to its average value.
However, in plasma sources wherein the field strength varies with position a higher demand is placed on the electric breakdown field strength of the dielectric material. A dielectric material that is sufficient for a uniform volume DBD plasma source need not be sufficient for such a plasma source. For example, the demands on the breakdown voltage increase significantly when a plurality of embedded electrodes is present in the dielectric layer, which make the electric field strength vary as a function of position. Surface DBD plasma sources are examples of plasma sources with such embedded electrodes.
FIG. 1b, illustrates an example of a surface DBD plasma source. Herein a first and second electrode 10, 12 are provided on opposite surfaces of the same dielectric layer 16. In this configuration, the gas-exposed surface of the dielectric layer 16 is not fully covered by the electrode 12 (or electrodes) at that surface. In such a structure, the electric field lines from the electrode 12 on the gas-exposed surface will run not only directly between the electrodes through the dielectric layer 16, but also along bent field lines, first through the gas space adjacent the dielectric layer 16 and from that gas space into the uncovered part of the surface of the dielectric layer 16. This gives rise to plasma on the gas-exposed surface, mostly along the edges of the electrode(s). Because no gas gap between the electrodes is involved, this type of plasma source is referred to as a surface DBD plasma source. Electric breakdown of the dielectric layer should be prevented everywhere, by using dielectric material with a breakdown strength that exceeds the highest electric fields in the structure, especially at and near the surface of the dielectric layer. This places a higher demand on the dielectric layer.
Breakdown can become more critical in another type of surface DBD plasma source, shown in FIG. 1c, wherein at least one of the electrodes is embedded in the dielectric layer. In such a configuration there are bent electric field lines that first run upwards from the embedded electrode(s), emerging from the gas-exposed surface of the dielectric layer 16 and elsewhere down back into the gas-exposed surface to another electrode. In such a configuration, the dielectric layer 16 needs to be very thin to ensure that sufficient electric field strength arises at the gas exposed surface of the dielectric layer 16.
A surface DBD plasma source of this type with embedded electrodes is disclosed in WO2010077138. This document mentions a ceramic coating as an example of a dielectric coating. The document mentions problems with reduced operational lifetime due to breakdown. This problem is addressed by switching off sections of the source that have become useless as a result of electric breakdown.
A plasma display that uses buried electrodes is disclosed in WO00/03956. The display has a front and back panel, between which plasma is excited at individual pixels. The back panel contains a buried electrode. Electrodes are provided in green tape (a flexible pre-fired ceramic) after application of bonding glaze to the metal core for bonding the core to the green tape. The bonding core is made by firing a glass-powder suspension at 550 centigrade. The structure is embossed to create ribs between different pixels and fired at 900 centigrade.
In practice, the manufacture of surface DBD plasma sources with large continuous areas of plasma has proved to be far more difficult than for volume DBD plasma sources. For example, manufacture is usually starts from a ceramic plate and applies electrodes to that plate. In this case, the ceramic plate needs to be thin (e.g. at most one or less than a few mm thick, e.g. up to 1.5 mm). Due to the brittleness of ceramics, this limits the size of the plates that can be used. Another problem with large thin layer can be mismatch of thermal expansion coefficients of different layers. As a result, the size of commercial surface DBD plasma sources have remained limited to substantially less than a meter, even if thermal expansion problems are reduced by cooling. For example, an oil bath may be used to cool an external electrode on the dielectric layer.